The prevalence of non-insulin dependent diabetes mellitus (NlDDM) increased markedly from 1930 to 1980 in the U.S. Early in this period the prevalence of NlDDM was lower in African Americans than in Caucasians. However, by 1989, the age-standardized prevalence of NIDDM was roughly 2- fold higher in blacks than in whites for both sexes. Because of the huge public health burden created by NIDDM, effective intervention strategies for preventing NIDDM, particularly among high risk groups such as African Americans, are urgently needed. Roughly 70 to 80% of people with impaired glucose tolerance or NIDDM in the U.S. have abdominal obesity with insulin resistance. In this context, the primary aim of this study is to evaluate the effectiveness of a multiple-intervention weight loss program in normalizing glucose tolerance in African Americans with abdominal obesity and IGT or mild NIDDM. The participants in the study will be 48 female and 48 male volunteers from among the 4155 African American employees of the Washington University Medical Center. The weight-loss program is designed to minimize physical discomfort and disruption of life-style, and to avoid major changes in the foods usually eaten, in order to maximize compliance. The intervention will begin with 7 days on a low calorie diet in order to put the subjects in negative caloric balance, rapidly initiate weight loss, and, we hypothesize, decrease insulin resistance and improve glucose tolerance. To maintain negative caloric balance and induce gradual, prolonged weight loss, after the initial 7 days on a low-calorie diet, the participants will (a) decrease the fat content of their diets by 100 to 150 kcal/day, i.e., 11 to 17 grams/day, (b) increase their energy expenditure by 100 to 150 kcal/day by incorporating small increases in physical activity into their daily routine, and (c) go on the low calorie diet for 2 days, or fast for one day, per month. A negative caloric balance of 200 kcal/day plus 2 days of the low calorie diet, or one day of fasting, per month results in a caloric deficit equivalent to a weight loss of approximately 10.8 kg of fat per year. The second aim is to evaluate the effectiveness of this program in maintaining improvement, or inducing further improvement, in glucose tolerance over 2 to 4 additional years. If the program induces major improvements in glucose tolerance, the third aim will be to determine the underlying physiological mechanisms by (a) evaluating the relationship between loss of visceral fat, measured by MRl, and improvement in glucose tolerance, (b) quantifying changes in insulin action using the hyperinsulinemic, euglycemic clamp procedure, and (c) determining the effect of the intervention on insulin secretion using the hyperglycemic clamp procedure.